The present invention relates to a semiconductor memory device having improved reliability which can prevent the occurrence of a soft error due to the incidence of alpha particles.
Semiconductor memory devices are generally categorized into dynamic and static type memory devices. In the dynamic type memory device, each memory cell is composed of a single capacitor and a single MOS (metal oxide semiconductor) transistor. In the static type memory device, each memory cell is composed of four transistors 11 to 14 and two load resistors 15 and 16. In FIG. 1, characters BL and BL indicate a pair of bit lines and WL a word line.
In the semiconductor memory device of the dynamic type, data is kept by storing a charge in the capacitor. For this reason, since the elements are very small, the quantity of stored charges is quite small. With such minute charges stored, the data tends to be destroyed upon external application of alpha particles.
The static type semiconductor memory device is constructed so as to hold data by always feeding current through a resistor (15 or 16 in FIG. 1). Because of this feature, in the static type semiconductor memory device, the occurrence of a soft error is less frequent than in the dynamic type semiconductor memory device. Nevertheless, miniaturization and low power dissipation of the elements of the semiconductor memory device increases the probability that a soft error will occur. More specifically, in a static type semiconductor memory device as shown in FIG. 1, as the degree of miniaturization increases, stray capacitances at the nodes N1 and N2 of load resistors 15 and 16 and drive transistors 11 and 12 become smaller. The integration density of a semiconductor chip reflecting the present stage of semiconductor technology allows the stray capacitance at the node N2 to be approximately 10 fF (10.times.10.sup.-15 F) and hence, a quantity of charges allowed to be stored at the node N2 is 50 fC (50.times.10.sup.-15 C) at the most. When alpha particles are incident on a semiconductor region corresponding to the node N2, a current I.sub..alpha. flows along a path between the node N2 and ground. As shown in FIG. 2, the current I.sub..alpha. is a pulsative current with a very narrow width, its peak 1 value is 300-400 .mu.A, and the total quantity of its charge reaches 100 fC. Thus, the total quantity of the charge of the current I.sub..alpha. is remarkably larger than the quantity of charge stored in the node N2. Therefore, the flow of the current into the node N2 results in destruction of the data stored therein. Further, the decreased power dissipation necessitates an extreme increase in resistance of the resistors 15 and 16 (up to the order of giga-ohms). The result is that the current flowing from the power source V.sub.CC into the node N2 becomes small, and hence, the quantity of charge stored in the node N2 also becomes small. This implies that the stored data is more easily destroyed by alpha particles.